There is a major concern in the steam-assisted gravity drainage (SAGD) process that the existence of thief zones such as top water and/or gas cap overlying the oil sand deposit has a detrimental effect on the oil recovery. The objective of this numerical study is to investigate the mechanisms that govern the oil and steam loss during the SAGD process in the presence of a top water zone or a gas cap.

The reservoir model, STARS, developed by the Computer Modelling Group (CMG) Ltd. was first validated based on history matching of two 3-D SAGD laboratory experiments with a top water zone and a gas cap, respectively. The experiments that were designed to mimic SAGD processes in the Athabasca region were conducted at field conditions of reservoir pressure and steam injection temperature. It was found that STARS was capable of history matching the top water and gas cap experiments. Therefore, it is believed that the numerical simulation captured the major mechanism of oil movement from the oil zone into the top thief zones as observed in the experiments.

Lab-scale numerical sensitivity study indicated that oil movement from the oil zone into the top thief zones occurred when a very small pressure gradient existed between the oil zone and the top thief zone (e.g. < 10 kPa/m). Higherpressure gradients resulted in more oil and steam movements into the top thief zone, less oil production and higher steam-oil ratio.


There is a major concern by Alberta oil producers that the production of natural gas in association with oil sands would lower reservoir pressure, reduce oil recovery and may be prohibit the economic recovery of any oil. Alberta Department of Energy (ADOE) and Alberta Energy and Utilities Board (AEUB) initiated a series of field-scale numerical modeling studies1,2 to assess the potential applicability of the steam-assisted gravity drainage (SAGD) oil recovery process under a variety of reservoir conditions such as reservoir thickness, reservoir depth, initial pressure, oil saturation, and the presence of top water zones and gas caps. It was found that top water zones and gas caps are thief zones to SAGD process. These thief zones have a detrimental effect on SAGD recovery performance especially when the pressure in the thief zones is reduced below optimum SAGD operating pressures due to natural gas production. Movement of oil into the top water zones and gas caps is simulated to occur. The volume of this oil seems to be generally proportional to the amount of outflow from the pattern due to thickness of the top water zones/gas caps and the pressure difference between the steam chamber and the top thief zones. SAGD process costs depend on the amount of steam that flows into the top water zones and gas caps, from which no oil is produced.

A case in point is the Surmont oil sands lease, which Gulf plans to develop with a SAGD pilot starting in June 1997 followed quickly by commercial development.

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